I am assuming you don't have any messages in your Event Log about the cause of the restart. It just blows up and restarts, yes? No blue screen or Dr. Watson error before the crash?
When I get this symptom, I generally start looking at memory and I/O problems. Bad memory can certainly cause random reboots. So can a bad SCSI card or terminator. Do you use IDE or SCSI? You might have also uncovered a problem with the dual processors that would not have been apparent under ME. A bad power supply can also cause random reboots.
The first thing to do is reseat everything in the box. This baby is getting a little elderly, so blow out the dust bunnies. You might try taking out one of the processors to see if that resolves the problem. If not, swap in the other processor. Make sure you get the voltage regulators installed correctly.
This was first published in June 2001